The present disclosure relates to manufacturing processes and systems for producing gypsum products from starting materials including calcined gypsum and water, and more particularly relates to a mixing apparatus for producing aqueous calcined gypsum slurry used in supplying gypsum slurry to a production line, for example a gypsum wallboard production line.
In many types of gypsum products, set gypsum (calcium sulfate dihydrate) is often a major constituent. For example, set gypsum is a major component of end products created by use of traditional plasters (e.g., plaster-surfaced internal building walls), and also in gypsum board employed in typical drywall construction of interior walls and ceilings of buildings. In addition, set gypsum is the major component of gypsum/cellulose fiber composite boards and products, as described in U.S. Pat. No. 5,320,677, for example. Typically, such gypsum-containing cementitious products are made by preparing a mixture of calcined gypsum (calcium sulfate alpha or beta hemihydrate and/or calcium sulfate anhydrite), water, and other components, as appropriate to form gypsum slurry. The gypsum slurry and desired additives are often blended in a continuous mixer, as described in U.S. Pat. No. 3,359,146, for example.
In a typical gypsum board manufacturing process, gypsum board is produced by uniformly dispersing calcined gypsum (commonly referred to as “stucco”) in water to form a dispersion of aqueous calcined gypsum. The aqueous calcined gypsum slurry is typically produced in a continuous manner by inserting stucco and water and other additives into a mixer which contains means for agitating the contents to form a uniform gypsum slurry. The slurry is continuously directed toward and through a discharge outlet of the mixer and into a discharge conduit connected to the discharge outlet of the mixer. Aqueous foam can be combined with the aqueous calcined gypsum slurry in the mixer and/or in the discharge conduit. A stream of foamed slurry passes through the discharge conduit from which it is continuously deposited onto a moving web of cover sheet material (i.e., the face sheet) supported by a forming table. The foamed slurry is allowed to spread over the advancing face sheet.
Various methods are known for producing foamed gypsum board having edges that are denser and harder than the core portion of the board, such as, by diverting a portion of the slurry from the mixing chamber. The diverted portion either contains a reduced amount of foam therein (and thus, is denser than slurry having a higher concentration of foam) or is then treated separately in one or more supplementary mixers with high agitation and/or defoaming agents to remove all or most of the foam and thus produce a harder, denser “edge” slurry to be cast at the edges of the cover sheet so that it comes into contact with the sides of the cast main slurry stream. Examples of such techniques are described in U.S. Pat. Nos. 2,985,219 and 4,279,673.
A second web of cover sheet material (i.e., the back sheet) is applied to cover the gypsum slurry and form a sandwich structure of a continuous wallboard preform. The wallboard preform is subjected to forming, such as at a conventional forming station, to obtain a desired thickness.
The calcined gypsum reacts with the water in the wallboard preform to form a matrix of crystalline hydrated gypsum or calcium sulfate dihydrate and sets as a conveyor moves the wallboard preform down the manufacturing line. The hydration of the calcined gypsum provides for the formation of an interlocking matrix of set gypsum, thereby imparting strength to the gypsum structure in the gypsum-containing product. The product slurry becomes firm as the crystal matrix forms and holds the desired shape.
After the wallboard preform is cut into segments downstream of the forming station at a point along the line where the preform has set sufficiently, the segments are flipped over, dried (e.g., in a kiln) to drive off excess water, and processed to provide the final wallboard product of desired dimensions. The aqueous foam produces air voids in the set gypsum, thereby reducing the density of the finished product relative to a product made using a similar slurry but without foam.
Prior devices and methods for addressing some of the operational problems associated with the production of gypsum wallboard are disclosed in commonly-assigned U.S. Pat. Nos. 5,643,510; 5,683,635; 6,494,609; 6,874,930; 7,007,914; and 7,296,919, which are incorporated by reference. The problem of lump formation in the mixer is a long-standing problem. When the calcined gypsum slurry exits the mixer containing lumps of gypsum and the slurry is fed to a board machine for introduction between paper cover sheets, the lumps of gypsum cause the paper sheets to break which requires stoppage of the board machine to remove the broken paper sheets and/or cleanup the gypsum slurry which may spill onto the board machine through the broken sheets.
U.S. Pat. No. 5,683,635 discloses the use of a device in the mixer commonly referred to as a “lump ring,” which aids the mixing action in the mixer and is intended to prevent lumps of gypsum from being discharged from the mixer with the calcined gypsum slurry The lump ring comprises at least one ring projecting from a surface to define a small circumferential gap between the ring and an adjacent surface to prevent lumps larger in size than the gap from passing radially outward of the ring to the discharge outlet of the mixer. The lump ring can include two rings, projecting from opposing surfaces to define a labyrinthine path.
The size of the gap can affect the volumetric flow rate of material exiting the mixing chamber and correspondingly the amount of material held within the ring. The gap is fixed once the machine is operating, and it determines how much slurry can pass from the slurry mixer. The gap also determines the amount of slurry retained within the volume defined by the ring. It is thought that the more slurry resident within the mixer during its continuous operation the less chance there is for a lump to develop within the mixer. If the gap is too small, it can be difficult to obtain the necessary slurry volumetric flow rate to produce thicker products at a desired line speed, forcing the operator to reduce the line speed to obtain the desired product thickness. If the gap is too large, the volume within the ring is not kept sufficiently full with gypsum slurry to help prevent lumps.
Conventionally, to adjust the gap, the mixer is shut down and opened up. Shims are manually inserted between the lump breaker ring and rotor to adjust the gap. The mixer is then sealed up again and returned into service.
There is a continued need in the art to provide additional solutions to enhance the production of gypsum products. For example, there is a continued need for techniques for helping to reduce the tendency for lumps to develop within the mixer while being able to produce gypsum products with a range of desired thicknesses in a manner that is quick and simple without requiring excessive labor and/or downtime.
It will be appreciated that this background description has been created by the inventor to aid the reader and is not to be taken as an indication that any of the indicated problems were themselves appreciated in the art. While the described principles can, in some aspects and embodiments, alleviate the problems inherent in other systems, it will be appreciated that the scope of the protected innovation is defined by the attached claims and not by the ability of any disclosed feature to solve any specific problem noted herein.